When a dog is eating or resting, the parasympathetic nervous system comes into play, causing the pupils to become smaller. It gets activated during periods of relaxation. On the other hand, the parasympathetic nervous system is nicknamed the “rest and digest” response. This is why frightened dogs often have dilated pupils. When the sympathetic nervous system is activated, the pupils dilate to allow more light into the retinas, enhancing vision. The sympathetic nervous system is often referred to as “fight or flight.” It’s the segment of the animal’s nervous system that gets triggered when faced with danger or intense physical activity. While lighting plays a role in changing pupil sizes, the sympathetic and parasympathetic nervous systems are also involved. When a disease or trauma disrupts the nerves or muscles controlling pupil size, one pupil can become abnormally dilated or constricted. Why Do Dogs Have Different-Sized Pupils in Anisocoria? Sudden onset of anisocoria is considered a medical emergency, especially if it occurs following trauma. While it can be harmless, that is not always the case. While many of these diseases tend to be chronic, if you notice anisocoria occurring suddenly, seek veterinary care as soon as possible. There are also a few conditions that can affect the brain, disrupting the necessary signals required for normal pupil constriction. Several diseases and conditions can impact these nerves and muscles, affecting the ability to constrict and dilate the pupils appropriately. It’s the muscles of the iris and the nerves that run through the eye, face, and brain that control how big the pupils are. The pupil is surrounded by the iris, the colored ring of the eye. ![]() When one pupil is notably larger than the other, it is called anisocoria. Normal, healthy pupils are symmetrical and, whether dilating or constricting, they change size simultaneously. Conversely, in bright sunlight, the pupil will constrict (get smaller), reducing the amount of light hitting the retina so that your dog is able to see clearly. In low-light conditions, a normal pupil will dilate (get larger) to let in more light, enabling your dog to perceive shadows and get around in dim surroundings. The pupil, a small dark circle at the center of the eye, can adjust its size to let light into the back of the eye to assist in vision. ![]() Anisocoria is a condition in which a dog’s pupils are different sizes.
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